1. Field of the Invention
The present invention generally relates to fluid control systems for controlling the extension and retraction of fluid-power actuators, or cylinders. More specifically, this invention relates to a flow control system which is capable of selectively operating a pneumatic cylinder between high and low pressures, such that the cylinder cooperates to engage and clamp a workpiece while under the influence of the lower pressure so as to minimize the impact on the workpiece, and thereafter be subjected to the higher pressure so as to increase the clamping load on the workpiece, wherein the pneumatic cylinder is particularly suitable for use in actuating a spot welding gun for the purpose of welding two or more relatively thin sheets of metal.
2. Description of the Prior Art
Fluid-power actuators, more commonly referred to simply as cylinders, are widely employed in both manual and automated operations where linear motion is useful. In automated operations, control systems for controlling fluid flow to and from a cylinder are typically necessary to either accurately limit the extension or retraction of the cylinder rod, or accurately control the stroking rate of the rod, or combinations thereof.
Cylinders used to actuate electric welding guns, such as resistance welding guns, are required to bring a pair of welding electrodes into contact with a workpiece to be welded. With the electrodes contacting the workpiece, the cylinders must apply a sufficiently high clamping pressure through the electrodes to the workpiece so as to ensure a proper weld. Generally, welding guns are often used on mass production assembly lines to permanently join two or more workpieces. Resistance welding guns are especially suited for use on mass production assembly lines, such as automated automobile assembly lines where body panels, doors, hoods, and their support structures are welded together to form subassemblies that, in turn, are subsequently welded to the frame or the unit construction of an automobile. Unit construction automobile bodies are themselves complex structures of stamped sheet metal sections which must be welded together to form a framework to which the outer body panels are attached.
Resistance welding is a process which, through the application of heat and pressure, coalesces two or more metal sheets or panels, without the use of fluxes or filler metals, to form a permanent joint. The necessary heat is generated by the resistance of the metal sheets to the flow of an electric current through the metal sheets. Conventional resistance welding guns include electrodes which serve as terminals for an electrical circuit. During the welding process, a pair of cylinders bring the electrodes into contact with opposite sides of the metal sheets to be welded so as to firmly clamp the workpieces together. An electric current is then passed through the electrodes and through the sheets.
The application of force through the electrodes is beneficial in that it assures a firm contact between the metal sheets being welding, and also assures that a sufficient electrical contact between the metal sheets and the electrodes is made. However, excessive force must be avoided where the metal sheets being welded are thin and, therefore, susceptible to deformation from excessive impact loads. This is particularly true in the automotive industry where the demand for lighter-weight vehicles has resulted in the use of sheet metal which has traditionally been about 0.035 inches thick, but more recently has become significantly thinner. In view of the high forces which must typically be applied by the cylinders to ensure short cycle times as well as sufficient contact between the electrodes and the sheet metal, deformation often becomes a reoccurring problem on automobile assembly lines.
Welding guns which are typically used on an automated assembly line typically include a pair of arms pivotably mounted to a base, with each arm being fitted with an electrode. A cylinder is attached to the movable arm to move the electrode into and out of position for welding. Due to the nature of the application, the movable electrode will typically come in contact with the workpiece prior to the fixed electrode as the cylinder is extended to bring the electrodes together. The initial impact is largely due to the high momentum and the associated dynamic forces required of the cylinder in order to quickly and firmly approach the metal sheet and clamp them together. The initial impact force is generally 2 to 4 times greater than the calculated force for any given cylinder. While this initial impact has not typically caused excessive deformation during assembly operations, with body panels in automobiles getting thinner, these loads have been found to cause unacceptable deformation of the body panels.
Several approaches for minimizing the adverse effect of the impact force generated by the cylinders have been suggested in the prior art. It has been suggested in the prior art that suitable results can be achieved by altering the construction of the cylinders themselves so as to reduce the actuation speed and initial clamping loads generated by the cylinders. However, a significant disadvantage to such an approach is that each welding operation may require a pair of cylinders which are specifically adapted for its purpose. As a result, not only are the cylinders relatively expensive in that they are customized to some degree, but the replacement of a cylinder on the assembly line mandates that the particular cylinder be kept in stock for each unique welding operation. Furthermore, it is not typically practical or desirable to reduce the actuation speed of the cylinders, in that slower speeds result in longer cycle times, which are highly undesirable in mass production operations.
Consequently, the prior art has often focused on flow control systems for operating a more or less standard pair of cylinders. Examples of the prior art directed to this approach include U.S. Pat. Nos. 3,497,660 to Henry-Biabaud, 4,579,042 to Neff, 4,680,441 to McKendrick, 4,733,042 to Nishiwaki et al., 5,032,704 to Neff et al., U.K. Patent Application 2,063,133 and Japanese Pat. 63-2575. Generally, each of the above seeks to operate a pair of cylinders under the influence of low pressure air until the electrodes are in proximity to or have contacted a workpiece, after which high pressure air is delivered to the cylinders to increase the clamping load on the workpiece prior to the actual step of welding the workpiece. While each of the above approaches may generally be capable of avoiding excessive impact of the electrodes against a workpiece, each shares the disadvantage of requiring a complicated and numerous assortment of control valves, pressure regulator valves, and/or computers with computer-interface control and sensing devices. As a result, initial cost and replacement costs can be rather high, as well as the time and costs involved in maintaining these complicated flow control systems.
From the above discussion, it can be readily appreciated that the prior art does not disclose an uncomplicated, low cost flow control system which is capable of regulating the manner in which a resistance welding gun is actuated to engage one or more workpieces, such that the electrodes of the welding gun engage and initially clamp the workpiece under the influence of a low impact force, and thereafter clamp the workpiece with a significantly higher clamping load.
Accordingly, what is needed is an economical flow control system which is capable of selectively operating a cylinder between high and low pressures, such that the cylinder cooperates to bring a corresponding pair of welding electrodes into engagement with one or more workpieces while under the influence of the lower pressure so as to minimize the impact on the workpieces, and such that the cylinder is thereafter subjected to the higher pressure so as to increase the clamping load on the workpieces. Such a flow control system would also be able to provide such capabilities with the use of a minimum number of flow control devices, such that the flow control device is relatively uncomplicated in construction and operation, so as to minimize costs and maintenance.